AWS RDS集群实例不需要相同大小，可以根据实际需求进行调整和配置。下面给出一个创建一个包含不同大小的RDS集群实例的示例代码：

resource "aws_rds_cluster" "example" {
  name           = "example"
  engine         = "aurora-mysql"
  engine_version = "5.7.12"
  database_name  = "mydb"
  master_username = "admin"
  master_password = "password"

  scaling_configuration {
    min_capacity = 1
    max_capacity = 4
    auto_pause   = true
  }

  cluster_identifier    = "example"
  availability_zones    = ["us-west-2a", "us-west-2b"]
  backup_retention_period = 7

  vpc_security_group_ids = [aws_security_group.example.id]

  tags = {
    Name = "example"
  }

  db_subnet_group_name = aws_db_subnet_group.example.name
}

resource "aws_rds_cluster_instance" "example" {
  cluster_identifier = aws_rds_cluster.example.id
  instance_class     = "db.t2.small"
  engine             = "aurora-mysql"

  availability_zone = "us-west-2a"

  tags = {
    Name = "example"
  }
}

resource "aws_rds_cluster_instance" "example2" {
  cluster_identifier = aws_rds_cluster.example.id
  instance_class     = "db.t2.medium"
  engine             = "aurora-mysql"

  availability_zone = "us-west-2b"

  tags = {
    Name = "example2"
  }
}

以上示例代码中，我们创建了一个name为'example”的RDS集群，其中包含了两个实例：一个使用db.t2.small实例类型，另一个使用db.t2.medium实例类型。这说明了不同大小的实例类型可以在同一个集群中共存。